1. Field of the Invention
The present invention relates to a catalytic distillation tower used for preparing dimethyl ether (DME), especially a dual-bed catalytic distillation tower. The present invention also relates to a method for preparing dimethyl ether using the same.
2. Description of the Prior Arts
Dimethyl ether (DME) is prepared by using different raw materials such as coal, nature gas, petroleum coke and biochar. The methanol is firstly prepared from the raw materials, and the methanol is dehydrated to obtain DME. In the dehydration reaction, catalysts are needed to carry out the reaction. Solid-state acid catalysts such as zeolite, silicone, aluminum oxide, resin etc. (Spivey, J. J., 1991) or modified acid ion exchange resin (U.S. Pat. No. 6,740,783) are commonly used in the dehydration. Resin catalysts are low temperature catalysts with reaction temperatures ranging from 70 to 150° C. and almost 100% conversion of methanol can be obtained. However, the low temperature catalysts cannot bear high temperature. Other catalysts are high temperature catalysts with reaction temperatures ranging from 200 to 350° C. When the reaction temperature is getting higher, however, methanol and DME will be further dehydrated to C2-C4 olefin, thereby resulting in the loss of catalyst activity and reduction of DME yield.
Further, to reduce the energy consumption during the catalytic distillation process, the catalysts are put in the catalytic distillation tower so the heat generated from the methanol dehydration reaction can be used for catalytic distillation. The requirement for this catalytic distillation process is that the catalytic reaction and distillation can happen at the similar tower pressure and temperature range.
US 2007/0066855 A1 discloses a method for production of DME using catalytic distillation tower so the heat generated from dehydration can be fully used and reduce the production cost. However, the tower temperature and pressure used for dehydration reaction result in that methanol and water in the stripping column cannot be separated easily. FIG. 1 shows the relative volatility of methanol and water at different tower pressures and can be used to calculate and estimate the difficulty for separating methanol and water at high tower pressure. FIG. 1 shows that the relative volatility of methanol and water reaches 1 (i.e. the azeotropic zone) when the mole fraction of liquid water equals 0.825 at a tower pressure of 18 bar. Azeotrope composition of methanol and water results in separation difficulties.
WO 2007/014534 provides high temperature dehydration in the catalytic distillation tower by using high temperature dehydration catalysts at a reaction temperature ranging from 160 to 180° C. and under a tower pressure ranging from 18 to 23 bar. However, the energy consumption of the condenser and reboiler increases in order to drive the catalytic reaction under high temperatures. U.S. Pat. No. 5,684,213 mentions high temperature dehydration reaction at a reaction temperature ranging from 350 to 400° C. and under 600 psi of tower pressure. When dehydration is carried out at such a high temperature, olefin side products increase to cause the decay of catalyst activity, resulting in lower DME yield. Hydrogen is added to the catalytic distillation tower to inhibit undesired side products generated. However, adding hydrogen increases the production cost and the reaction complexity.
The technique of using low temperature dehydration catalysts in the catalytic distillation tower is also disclosed. For example, WO 2007/014534 provides that dehydration reaction is carried out at a reaction temperature ranging from 130 to 158° C. and under a tower pressure ranging from 10 to 18 bar by using low temperature acid ion exchange catalysts. However, the low temperature catalysts can only be used at temperature lower than 140° C. When the reaction is carried out at a low temperature and pressure, the dew point of high purity DME at the top of the tower will decrease. For example, the dew point of DME becomes 40.3° C. when the tower pressure is as low as 9 bar and therefore using 20° C. industrial water to condense DME will be impractical. When the tower pressure is adjusted to 12 bar, the temperature at the catalytic column will be higher than 150° C., and the low temperature catalysts will be damaged.
All the above-mentioned methods use single-bed catalytic system in the catalytic distillation tower, i.e. only high temperature or low temperature dehydration reaction is carried out. In the conventional methods, the catalytic distillation tower cannot be easily adjusted depending on the grades of the feeding methanol and of the obtained DME because none of the conventional methods can provide suitable and complementary tower temperature and pressure conditions.